As a usual method, a paper-made container (hereinafter referred to as “cartons”) is widely used for packing liquid food like milk and juice. A sheet material for the carton is made by combining a cardboard made from pulp or similar raw material with a thin layer of heat-sealable plastic thereon or instead with a layer of aluminum foil thereon to keep liquid food in a sealed state for more extended period of time. These layers are laminated on at least such a surface of the cardboard as will become inner surface of a carton to which liquid food contacts when the cardboard is formed into the carton.
As commonly known, carton-packed food is shipped from a packing plant in the following manner: A sheet material in roll loaded on a payoff device is fed continuously into a carton forming line, in which line the sheet material is sized as designed and then folded into a desired shape of carton by a folding means. In the successive process, liquid food is filled into the carton thus formed and the filling port of the carton is sealed to complete packing process.
The sheet material for cartons is sterilized while travelling from the payoff device to the cartoning line and is transferred aseptically, a germfree condition without contamination, into the subsequent process.
Sterilization of sheet material conventionally used chemicals. This method however requires large scaled installations. Moreover, such method involves an anxiety about chemical residue on the sheet material sterilized. This anxiety therefore demands a process for a complete removal of chemicals from the sheet material. Consequently, an improvement has been desired in a sterilization device applicable to liquid food handling.
As a solution to this problem, an invention has been disclosed in JP2004-524895A1 (Patent Literature 1), which is known as a late alternative to a chemical sterilization apparatus for a sheet material. In Patent Literature 1, a unit for sterilization is proposed. The proposed unit is feasible for downsizing and efficiently sterilizes both surfaces of a sheet material with electron beam generated from an electron beam irradiation means while the sheet material is in high-speed transferring.
More specifically, the electron beam irradiation unit for sterilizing a sheet material defined in Patent Literature 1 has two electron beam irradiation means arranged across the sheet material facing each other in a part of a conveying path in the atmosphere. This arrangement radiates electron beam from each of the electron beam irradiation means to the sheet material in a high-speed transferring to sterilize both surfaces of the sheet material with electron beam. The sheet material thus sterilized is then transferred to the next stage of processing. Each of the electron beam irradiation means that generates low-energy electron beam accelerated at a voltage below 100 kV has its electron beam source enclosed by a vessel and has radiation shielding material such as lead plate installed inner wall of the vessel as a measure against X-rays that the electron beam source emits.
The above-stated electron beam irradiation unit for sterilizing a sheet material of a conventional style is able to efficiently sterilize both surfaces of sheet material. The unit however has a problem in that the unit is not compatible with such an electron beam irradiation means as works on a lower energy because such unit uses the electron beam irradiation means in the atmosphere in which electron beam having larger energy to certain degree is essential to sterilize.
Further, an electron beam irradiation means used in the atmosphere must have a radiation shielding material to shield X-rays emitted from the electron beam source of the irradiation means. This requirement results in a size-growth of the sterilization equipment as a whole possibly preventing the right positioning of the irradiation means on the sheet material conveying path.
Moreover, an electron beam irradiation means for use in the atmosphere easily generates plasma, which produces ozone (O3) from oxygen in the air.
The produced ozone, adhering to the sheet material sterilized, makes the sheet smell like ozone. Such smelling sheet material is not appropriate to a carton for liquid food packing. To remove the ozone odor, a measure such as lowering the oxygen concentration in the irradiation chamber by blowing nitrogen gas thereinto while electron beam irradiation is necessary; this arrangement makes the sterilization equipment sophisticated.
The object of the present invention is to provide an electron beam irradiation apparatus for sterilization of a sheet material. The device efficiently sterilizes at least one surface of a sheet material for a carton using a low-energy electron beam irradiation means under an appropriately regulated reduced-pressure atmosphere generating no ozone.